fluent 13.0 lecture06 turbulence 21 22

8/10/2019 Fluent 13.0 Lecture06 Turbulence 21 22

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Introduction to ANSYS FLUENT

Example in predicting near-wall cell sizeANSYSCustomer Training Material

• During the pre-processing stage, you will need to know a suitable size for the first

layer of grid cells (inflation layer) so that Y+is in the desired range.

• The actual flow-field will not be known until you have computed the solution (and

indeed it is sometimes unavoidable to have to go back and remesh your model on

account of the computed Y+values).

• To reduce the risk of needing to remesh, you may want to try and predict the cell

size by performing a hand calculation at the start. For example:

The question is what

height (y) should the first

row of grid cells be. We

> will use SWF, and are aiming for Y+ * 50

• For a flat plate, Reynolds number ( Re/ = ) gives Re, = 1.4x106

(Recall from earlier slide, flow over a surface is turbulent when ReL > 5x105)

Air at 20 m/sp = 1.225 kg/m3

[i = 1.8x10-5 kg/ms Flat plate, 1m long

AN SYS , Inc. Pro prie tar y

© 2010 ANSYS, Inc. All rights reserved. L6-21Release 13.0

December 2010




Example in predicting near-wall cell size [2]ANSYSCustomer Training Material

A literature search suggests a formula for the skin friction on a plate1thus:

C f = 0.058Re^0'2 <^=0.0034

Use this value to predict the wall shear stress t w

t w = 1 Cf p U l rw= 0.83 kg/ms2

UT=hi Ur = 0.82 m s

From recompute the velocity U

PRearranging the equation shown previously for y+ gives a formula for the

first cell height, y, in terms of U r

y+n y =-----

y=9xl0~4 mUT P

• We know we are aiming for>>+ of 50, hence:

our first cell height y should be approximately 1 mm.

1An equivalent formula for internal flows, based on the pipe-diameter Reynolds number is Cy = 0.079Re^0 25



December 2010




Other options for Wall Modelling Customer Training Material

Enhanced Wall Treatment Opt ion (GUI)

Combines a blended law-of-the wall and a two-layer

zonal model.

- Suitable for low-Re flows or flows with complex

near-wall phenomena.

Generally requires a fine near-wall mesh capable of

resolving the viscous sublayer (y+ < 5, and a

minimum of 10-15 cells across the “innerlayer”(viscous sublayer, buffer and log-law layers)

Scalable Wall Functions (TUI)

In practice, many users often fail to maintain

30 < < 300

Iteration by iteration, the first cell may change frombeing either inside / or outside of the viscoussublayer, which can lead to instabilities.

Scalable wall functions can be accessed by a TUIcommand/define/models/viscous/near-wall-

treatment/scalable-wall-functions

oute

inne

r layer

r layer

i - i

IX)

buffer &

sublayer



December 2010




Summary on Near-Wall Treatment/UNISYSCustomer Training Material

• Wall Functions are still the most affordable boundary treatment for many industrial

CFD applications

• Standard wall functions work well with simple shear flows, and non-equilibrium wall

function improves the results for flows with stronger pressure gradient andseparation

• Enhanced wall treatment is used for more complex flows where log law may not

apply (for example, non-equilibrium wall shear layers or the bulk Reynolds number

is low)



December 2010

fluent 13.0 lecture06 turbulence 21 22

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